civicaml/phases/constant_propagation.ml

(**
 * The compiler sometimes generates variables of the form foo$1, to make sure
 * that expressions are only executed once. In many cases, this leads to
 * over-complex constructions, for example when converting for-loops to
 * while-loops. We use the knowledge of these variables being constant by
 * propagation the constant values to their occurrences, and then apply
 * arithmetic simplification to operators to reduce the size and complexity of
 * the generated code. Note that this can only be applied to constants. For
 * variables in general, some form of liveness analysis would be required (e.g.
 * Static Single Assignment form). Expressions can only be propagated when they
 * have no side effects, i.e. when they do not contain function calls.
 *)
open Types
open Util

let is_const_name name =
    Str.string_match (Str.regexp "^.+\\$\\$[0-9]+$") name 0

let is_const = function Const _ -> true | _ -> false

let eval_monop = function
    | (Not, Const (BoolVal  value, _), ann) -> Const (BoolVal  (not value), ann)
    | (Neg, Const (IntVal   value, _), ann) -> Const (IntVal   (-value), ann)
    | (Neg, Const (FloatVal value, _), ann) -> Const (FloatVal (-.value), ann)
    | (op, opnd, ann) -> Monop (op, opnd, ann)

let eval_binop = function
    (* Arithmetic *)
    | (Add, Const (IntVal left, _), Const (IntVal right, _), ann) ->
        Const (IntVal (left + right), ann)
    | (Add, Const (FloatVal left, _), Const (FloatVal right, _), ann) ->
        Const (FloatVal (left +. right), ann)

    | (Sub, Const (IntVal left, _), Const (IntVal right, _), ann) ->
        Const (IntVal (left - right), ann)
    | (Sub, Const (FloatVal left, _), Const (FloatVal right, _), ann) ->
        Const (FloatVal (left -. right), ann)

    | (Mul, Const (IntVal left, _), Const (IntVal right, _), ann) ->
        Const (IntVal (left * right), ann)
    | (Mul, Const (FloatVal left, _), Const (FloatVal right, _), ann) ->
        Const (FloatVal (left *. right), ann)

    | (Div, Const (IntVal left, _), Const (IntVal right, _), ann) ->
        Const (IntVal (left / right), ann)
    | (Div, Const (FloatVal left, _), Const (FloatVal right, _), ann) ->
        Const (FloatVal (left /. right), ann)

    | (Mod, Const (IntVal left, _), Const (IntVal right, _), ann) ->
        Const (IntVal (left mod right), ann)

    (* Relational *)
    | (Eq, Const (IntVal left, _), Const (IntVal right, _), ann) ->
        Const (BoolVal (left = right), ann)
    | (Eq, Const (FloatVal left, _), Const (FloatVal right, _), ann) ->
        Const (BoolVal (left = right), ann)

    | (Ne, Const (IntVal left, _), Const (IntVal right, _), ann) ->
        Const (BoolVal (left <> right), ann)
    | (Ne, Const (FloatVal left, _), Const (FloatVal right, _), ann) ->
        Const (BoolVal (left <> right), ann)

    | (Gt, Const (IntVal left, _), Const (IntVal right, _), ann) ->
        Const (BoolVal (left > right), ann)
    | (Gt, Const (FloatVal left, _), Const (FloatVal right, _), ann) ->
        Const (BoolVal (left > right), ann)

    | (Lt, Const (IntVal left, _), Const (IntVal right, _), ann) ->
        Const (BoolVal (left < right), ann)
    | (Lt, Const (FloatVal left, _), Const (FloatVal right, _), ann) ->
        Const (BoolVal (left < right), ann)

    | (Ge, Const (IntVal left, _), Const (IntVal right, _), ann) ->
        Const (BoolVal (left >= right), ann)
    | (Ge, Const (FloatVal left, _), Const (FloatVal right, _), ann) ->
        Const (BoolVal (left >= right), ann)

    | (Le, Const (IntVal left, _), Const (IntVal right, _), ann) ->
        Const (BoolVal (left <= right), ann)
    | (Le, Const (FloatVal left, _), Const (FloatVal right, _), ann) ->
        Const (BoolVal (left <= right), ann)

    (* Logical *)
    | (And, Const (BoolVal left, _), Const (BoolVal right, _), ann) ->
        Const (BoolVal (left && right), ann)
    | (Or, Const (BoolVal left, _), Const (BoolVal right, _), ann) ->
        Const (BoolVal (left || right), ann)

    | (op, left, right, ann) -> Binop (op, left, right, ann)

let rec propagate consts node =
    let propagate = propagate consts in
    match node with

    (* Constant assignments are added to constants table *)
    | Assign (name, None, value, ann) when is_const_name name ->
        let value = propagate value in
        if is_const value then (
            Hashtbl.add consts name value;
            DummyNode
        ) else
            Assign (name, None, value, ann)

    | VarLet (dec, None, value, ann) when is_const_name (nameof dec) ->
        let value = propagate value in
        if is_const value then (
            Hashtbl.add consts (nameof dec) value;
            DummyNode
        ) else
            VarLet (dec, None, value, ann)

    (* Variables that are in the constant table are replaced with their constant
     * value *)
    | Var (name, None, ann) when Hashtbl.mem consts name ->
        Hashtbl.find consts name
    | VarUse (dec, None, ann) when Hashtbl.mem consts (nameof dec) ->
        Hashtbl.find consts (nameof dec)
    | Dim (name, ann) when Hashtbl.mem consts name ->
        Hashtbl.find consts name

    (* Apply arithmetic simplification to constant operands *)
    | Monop (op, opnd, ann) ->
        let opnd = propagate opnd in
        if is_const opnd
            then eval_monop (op, opnd, ann)
            else Monop (op, opnd, ann)

    | Binop (op, left, right, ann) ->
        let left = propagate left in
        let right = propagate right in
        if is_const left && is_const right
            then eval_binop (op, left, right, ann)
            else Binop (op, left, right, ann)

    | Cond (cond, texp, fexp, ann) ->
        let cond = propagate cond in
        let texp = propagate texp in
        let fexp = propagate fexp in
        (match cond with
        | Const (BoolVal value, _) -> if value then texp else fexp
        | _ -> Cond (cond, texp, fexp, ann)
        )

    | TypeCast (ctype, value, ann) ->
        let value = propagate value in
        (match (ctype, value) with
        | (Bool,  Const (BoolVal  value, _)) -> Const (BoolVal value, ann)
        | (Bool,  Const (IntVal   value, _)) -> Const (BoolVal (value != 1), ann)
        | (Bool,  Const (FloatVal value, _)) -> Const (BoolVal (value != 1.0), ann)
        | (Int,   Const (BoolVal  value, _)) -> Const (IntVal (if value then 1 else 0), ann)
        | (Int,   Const (IntVal   value, _)) -> Const (IntVal value, ann)
        | (Int,   Const (FloatVal value, _)) -> Const (IntVal (int_of_float value), ann)
        | (Float, Const (BoolVal  value, _)) -> Const (FloatVal (if value then 1. else 0.), ann)
        | (Float, Const (IntVal   value, _)) -> Const (FloatVal (float_of_int value), ann)
        | (Float, Const (FloatVal value, _)) -> Const (FloatVal value, ann)
        | _ -> TypeCast (ctype, value, ann)
        )

    | _ -> transform_children propagate node

let rec prune_vardecs consts = function
    | VarDec (_, name, _, _) when Hashtbl.mem consts name -> DummyNode
    | node -> transform_children (prune_vardecs consts) node

let phase = function
    | Ast node ->
        let consts = Hashtbl.create 32 in
        let node = propagate consts node in
        Ast (prune_vardecs consts node)
    | _ -> raise (InvalidInput "constant propagation")